Halftone screening is a process by which shades of gray can be represented in images formed with a bi-level marking system, such as black ink on white paper. As is the case in most printing systems, whether color or black and white, the absence or presence of the ink on the paper is used to represent the printed image. In order to represent halftones (shades between the presence or absence of the printed ink), the original image is screened to produce a pattern, made up of variable size dots, which appear to the human eye as a halftone image. Thus to prepare a photograph for printing, it is necessary to perform the process of halftone screening. which converts the continuous gray shades of the original into dots of varying size and shape.
Originally, photomechanical screening systems used physical photographic screens. In computer based screening systems, images are typically stored in a memory by representing tone values for each pixel of the original image. For a black and white image, the stored pixels represent the grayscale value corresponding to each pixel. For a color image, each color plane is stored as an array of pixels each representing the tone value for each pixel of the image in each respective color plane. For example, if each of the pixels of a black and white image is represented by an 8 bit digital word, then the tone value for a given image pixel is one of 256 values between the black level and the white level, with 0 corresponding to white and all ones (or 255), corresponding to black.
Digital screening devices and digital raster scan printing devices simulate the generation of a halftone cell to produce a screened image. The most common form of digital screening is to generate a repeating spot function corresponding to the halftone cell, and compare each pixel of the grayscale image with a corresponding pixel in the repeating spot function. The period and angle of the repeating spot function correspond to that of the resulting grid of dots, while the shape of the surface defined by a single repeating halftone cell spot function determines the shape of the resulting dots for each shade of gray.
There are also many screening methods that make use of a screen pattern cell, which can be a one dimensional strip. a band, or any two dimensional area, such as a square, that contains a section of the final screen spanning more than one halftone cell. The screening method will repeatedly generate the screen pattern cell, resulting in a final screen of the desired ruling and angle. The critical step in halftone screening is the generation of the screen pattern cell which will be repeated to form the final screen covering the entire input image area. Examples of prior art techniques include rational tangent (U.S. Pat. No. 4,149,194 to Holladay), rational supercell (European patent document 0 427 380 A.sub.2, corresponding to U.S. Pat. No. 5,235,435), irrational tangent (U.S. Pat. Nos. 4,499,489 to Gall et al, and 4,350,996 to Rosenfeld). In addition, three alternative screening techniques described in three patents by the inventor of the present invention, namely U.S. Pat. No. 5,291,310, U.S. Pat. No. 5,307,181, and U.S. Pat. No. 5,315,406, may each be used to generate a suitable screen pattern cell.
Digital halftone screening consists of first generating a spot function or screen pattern cell. The spot function is usually stored as a fairly small pattern that repeats itself or is repeatedly generated by programming, At any point where the original image is greater than the spot function, the output is marked. At any point where the image is not greater than the spot function, the output is not marked. In other words, if the value of the image pixel is greater than corresponding value of the spot function, a mark is generated by the marking engine, whereas if the value of the image pixel is less or equal to the spot function value, then no mark is generated by the marking engine, or vice versa. In this way, the final screened image, composed of dots, is produced. In color printing, there are four separate steps of halftone screening, one each for the cyan, magenta, yellow, and black inks.
Much prior art work has been devoted to producing screens which will produce good results and avoid artifacts in the final image. One such artifact to be avoided is the moire pattern. There are a number of sources of moire in the halftoning process. One source of moire is the moire between the screen girds used for the different ink planes in a color image. To counteract the tendency for moire patterns to result, color screens are angled at 0 (yellow), 15 (cyan), 75 (magenta) and 45 degrees (black). If these angles are adhered to precisely, as well as the screen ruling being precisely identical for all four planes, then optimum results (minimum moire) are achieved. Screening at these angles in photomechanical screening systems is achieved by rotating the photographic screen carriers. In digital screening devices and digital raster scan imaging devices, however, there is some difficulty in exactly reproducing irrational numbers, such as the tangent of 15 degrees or the tangent of 75 degrees. (Rational numbers can be represented as the ratio of two integers; irrational numbers are endless non-repeating decimals). Both rational tangent, and irrational tangent digital screening systems are known.
In general, the use of a repeating spot function, or screen pattern cell, for screening does work, and is used for the overwhelming majority of printing today. Aside from simplicity of implementation, the great strength of a repeating spot function, is its ability to render areas of constant or smoothly varying gray value. Many attempts to improve various aspects of the halftoning process (including error diffusion and so-called "stochastic" techniques) abandon the precise repetition of identically shaped dots, thereby introducing a grainy appearance.
Nonetheless, the quest for the highest quality image continues. Traditional halftoning techniques suffer from one persistent problem, that of unwanted moire patterns caused by the interference between patterns in the original grayscale image and the generated spot function, which could be called subject moire. (A different source of artifacts in digital raster scan screening systems is screener induced moire, which results from the interaction between the screen and the finite resolution of the output image forming device). Screener induced moire artifacts are more pronounced when using fine screen rulings, low output resolutions, or a combination of both. As a result, it is typical to use a 3500 dot per inch (dpi) output resolution to minimize screener induced moire artifacts and obtain a quality image.